This invention is generally related to imaging objects, such as machine-readable symbols or bar codes.
A variety of methods exist for tracking and providing information about items. For example, inventory items typically carry printed labels providing information such as serial numbers, price, weight and size. Some labels include data carriers in the form of machine-readable symbols that can be selected from a variety of machine-readable symbologies, such as barcodes, area or matrix codes and stacked codes. The amount of information that the symbols can contain is typically limited by the space constraints of the label. These symbologies typically rely on patterns of light and dark symbol elements to encode data. For example, barcode symbologies employ patterns of alternating dark elements (e.g., bars) and light symbol elements (e.g., spaces). Information is encoded in the width of the alternating bars and spaces. The use of only two types of symbol elements (i.e., light and dark) limits the amount of information that can be encoded in a symbol of a given length (density).
Bar code readers typically fall into two classes: scanners and imagers Scanners typically employ a laser that sweeps through a field of view using mechanical beam defection devices and provide great depth of field. Mechanical beam deflection devices include dithering mirrors, rotating polygons and galvanometers, as well as other technologies to deflect the laser beam across the field of view. Imagers employ one- or two-dimensional arrays of photodetectors to effectively produce a one-or two-dimensional image of the field of view. Imagers employing photodetector arrays typically project monochrome light on an image surface, where the photodetector array receives light reflected therefrom.
Unfortunately, laser scanners are large in size, heavy and suffer from poor shock resistance. Furthermore, laser scanners suffer from poor reliability, low life, high power consumption and vulnerability to vibration, primarily due to the laser and laser scanning components, including the mechanical devices used to scan the laser beam across the field of view. To maximize the depth of field for imagers or other photodetector array readers, small effective apertures must be used. However, small effective apertures significantly limit the overall efficiency of these readers and result in increased power consumption, greater weight and larger size than is desirable.